Like a 5.1 channel or 7.1 channel stereo system, an audio system having a plurality of sound signal channels and loudspeakers that provides a high quality sound space has come into wide use. In such a high quality audio system, it is extremely difficult for a user to appropriately adjust by him- or herself frequency and phase characteristics of reproduced sounds of respective channels, delivered from a plurality of loudspeakers such that the characteristics are suited for the sound field and thereby obtaining an optimum sound space that gives highly realistic sensations. For this reason, such an audio system is provided with a so-called automatic sound field correcting system, which automatically creates an optimum sound space by correcting sound field characteristics on the system's side.
As this kind of automatic sound field correcting system, a conventional art disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-151402 or United States Patent Application Publication No. 2005/0137859 has been previously known. In this conventional art, a test signal such as a pink noise is outputted from the loudspeaker of each of the channels. The test signal is collected by a microphone and a sound pressure level thereof is measured. Based on the measurement data thus obtained, frequency and phase characteristics and the like of the sound field are calculated, and various parameters of a sound field correcting equalizer provided for each of the channels are adjusted. A sound field correction is thus performed.
To be more specific, in each of the channels the audible frequency band is divided into nine frequency bands, and the sound field correction is performed by using a fixed frequency band graphic equalizer (hereinafter referred to as “GEQ”) having nine bands (63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz). The selectivity factor (Q-factor) of these GEQs is suppressed to a relatively low value in order to prevent phase differences of sound signals from increasing among the channels even if equalizing characteristics are set differently in the respective channels.
Also, correspondingly to the characteristics of the GEQ, a band pass filter (hereinafter referred to as “BPF”) with nine bands having low selectivity (Q-factor) is used as a BPF for analyzing sound pressure of the test signal collected by the microphone.
As described above, in the sound field correction according to the conventional art, a BPF or GEQ with low selectivity factor (Q-factor) is used in the measuring or correcting step. Therefore, the frequency resolution provided at the time of measuring or correcting is not high enough for a peak occurring in a narrow band, such as a peak generated by a standing wave due to low-frequency signal components. Consequently, when a measurement or correction is performed using such a BPF and GEQ, there have been a problem that suppression of a peak level can be achieved, however, surplus correction is performed on a spectrum of a broader band including the peak, and thus the frequency characteristics of a channel concerned are distorted.
In contrast, by using a so-called parametric equalizer, wherein a central frequency or the selectivity factor (Q-factor) thereof can be arbitrarily adjusted, it becomes possible with relative ease to follow a peak occurring in a narrow band generated by the standing wave, and an appropriate correction can be performed. However, a parametric equalizer has a problem that the equalizer generally has high selectivity factor (Q-factor) and reproduction of an ideal sound field is difficult to achieve due to disarrangement in the phase relationship among the respective channels that is caused when filters with different characteristics are inserted into the respective channels.